Monterey Bay National Marine Sanctuary research director Andrew Devogelaere said the spinach-looking seaweed is an algae called ulva. Ulva washes onshore from time to time, but this time, "There is just a lot more than anyone has ever seen before," Devogelaere said.

Scientists were busy hypothesizing why so much appeared Tuesday. One hypothesis is that unusually hot July weather and intense sunlight caused the bloom.

"It could be that there's more light reaching the bottom. And it's a flat, calm, sandy place where this kind of algae can grow really fast," Devogelaere said.